[Radio Wrangler]

It multiplies the circuit current by its voltage which gives true instantaneous power, but if the circuit is running on AC, the power value will fluctuate more rapidly than the pointer can follow, so the pointer will show the true mean power.

'RMS power' is a bit of loose terminology that's very common. It's mostly harmless, but this is exactly the situation where it causes confusion. It's simply mean power that does the work, and that's in watts. RMS values only make sense for voltage or for current and RMS is the equivalent DC voltage (or current) that gives you the same mean power.

The power meter has TWO coils and no permanent magnet. One coil provides the fixed field... connectedd across a shunt resistor, sensing the circuit current. The moving coil is driven by a small current from a burden resistor, sensing the circuit voltage.

The torque on the moving coil is proportional to the fixed field strength (circuit current) times the moving coil current (circuit voltage) and the coil moves against the spring proportionally to the torque. Volts times amps equals watts, and voila! the meter produces a deflection proportional to the power in the circuit!

David